There's a question in my regression class I'm having trouble with. We are basically given 6 points (5,y1), (5,y2), (10,y3), (10, y4), (15,y5), (15,y6)

and we are asked how it is possible to estimate the error term variance without fitting a regression line.

I'm thinking maybe the solution has something to do the with formula for sample variance? But I really don't know. Can anyone help?